The present invention relates generally to control circuitry for occupancy detectors, and is more particularly directed to false triggering of occupancy detectors caused by transient signals.
An occupancy detector is a device used in automatic light switches and security systems to turn on a light or to activate some other form of alarm or warning indicator when a person or motor vehicle enters a monitored area. Such devices are used with outdoor residential lighting, for example, to illuminate a walkway as a person approaches the front door or to illuminate a driveway as a car approaches. They are also referred to as intrusion detectors, particularly in security applications.
A popular form of occupancy detector, for example, is the passive infrared (PIR) motion detector. The PIR motion detector functions by sensing heat in the form of infrared radiation emanating from a person or similarly warm object as the person or object enters or moves about in the field of view of the detector. When the motion detector detects an appropriate heat impulse characteristic of a moving heat source, it provides an electrical signal to activate the light or other alarm. For electric lighting it is common for the motion detector to keep the light on for a pre-set duration after motion has been detected. Thus, for example, when a person enters the motion detector's field of view, a light comes on and remains on typically for five, ten or fifteen minutes at the selection of the user. The light is then automatically extinguished at the end of the pre-set period unless further motion is detected.
A problem arises, however, when the light is extinguished. The sudden drop in electrical current through the light as the light is turned off can cause extraneous transient electrical noise signals in other parts of the motion detector circuitry. These transient noise signals sometimes trigger the motion detector to turn the lights back on again. The lights may also be re-triggered by infrared radiation from the lights themselves as they are extinguished. Typical lights require a few moments to cool down after being extinguished. When the lights cool through a certain temperature range, they emit an increased proportion of infrared radiation, which, depending on the disposition of the motion detector in its surroundings, may be picked up by the motion detector, either directly or through reflections. The motion detector may then interpret the transient infrared signal as an occupancy event and re-trigger the lights.
To counter these problems, motion detectors in the past have included timing circuitry for disabling the motion detector mechanism for several seconds as the lights are extinguished. During those several seconds the lights are fully extinguished and cool down and the extraneous noise signals dissipate. The motion detector then resumes scanning for further motion.
In the past, motion detectors have included specialized integrated-circuit timing chips or other complicated circuitry for momentarily disabling the motion detector or otherwise addressing the false triggering problem as the lights are extinguished. For low-cost PIR motion detectors the extra cost of the timing and disabling circuitry is significant and increases the price at which the motion detector may be offered to the end user.